Much database searching is based on the principles of Boolean logic. Boolean logic refers to the logical relationship among search terms, and is named for the British-born Irish mathematician George Boole.
Boolean logic consists of three logical operators:
OR
AND
NOT
Each operator can be visually described by using Venn diagrams, as shown below.
OR logic
Venn diagram for OR
college OR university Question: I would like information about college.
In this search, we will retrieve records in which AT LEAST ONE of the search terms is present. We are searching on the terms college and also university since documents containing either of these words might be relevant.
This is illustrated by:
the shaded circle with the word college representing all the records that contain the word "college"
the shaded circle with the word university representing all the records that contain the word "university"
the shaded overlap area representing all the records that contain both "college" and "university"
OR logic is most commonly used to search for synonymous terms or concepts.
Here is an example of how OR logic works:
Search terms
Results
college
396,482
university
590,791
college OR university
819,214
ORlogic collates the results to retrieve all the unique records containing one term, the other term, or both of them.
The more terms or concepts we combine in a search with OR logic, the more results we will retrieve.
Venn diagram for OR
college OR university OR campus
For example:
Search terms
Results
college
396,482
university
590,791
college OR university
819,214
college OR university OR campus
929,677
Venn diagram for AND
poverty AND crime Question: I'm interested in the relationship between poverty and crime.
In this search, we retrieve records in which BOTH of the search terms are present
This is illustrated by the shaded area overlapping the two circles representing all the records that contain both the word "poverty" and the word "crime"
Notice how we do not retrieve any records with only "poverty" or only "crime"
Here is an example of how AND logic works:
Search terms
Results
poverty
76,342
crime
348,252
poverty AND crime
12,998
The more terms or concepts we combine in a search with AND logic, the fewer results we will retrieve.
Venn diagram for AND
poverty AND crime AND gender
For example:
Search terms
Results
poverty
76,342
crime
348,252
poverty AND crime
12,998
poverty AND crime AND gender
1,220
In addition: a very few search engines make use of the proximity operator NEAR. A proximity operator determines the closeness of terms within the text of a source document. NEAR is a restrictive AND. The closeness of the search terms is determined by the particular search engine. Most search engines default to proximity searching by default.
NOT logic
Venn diagram for NOT
cats NOT dogs Question: I want information about cats, but I don't want to see anything about dogs.
In this search, we retrieve records in which ONLY ONE of the terms is present, the one we have selected by our search
This is illustrated by the shaded area with the word cats representing all the records containing the word "cats"
No records are retrieved in the area overlapping the two circles where the word "dogs" appears, even if the word "cats" appears there too
Here is an example of how NOT logic works:
Search terms
Results
cats
86,747
dogs
130,424
cats NOT dogs
65,223
NOT logic excludes records from your search results. Be careful when you use NOT: the term you do want may be present in an important way in documents that also contain the word you wish to avoid. For example, consider a Web page that includes the statement that " cats are smarter than dogs." The search illustrated above would exclude this document from your results.
Combined AND and OR logic
Question: I want information about the behavior of cats. Search: behavior AND (cats OR felines)
You can combine both AND and OR logic in a single search, as shown above.
The use of parentheses in this search is known as forcing the order of processing. In this case, we surround the OR words with parentheses so that the search engine will process the two related terms as a unit. The search engine will use AND logic to combine this result with the second concept. Using this method, we are assured that the semantically-related OR terms are kept together as a logical unit.
Boolean Operators Tutorial
Much database searching is based on the principles of Boolean logic. Boolean logic refers to the logical relationship among search terms, and is named for the British-born Irish mathematician George Boole.
Boolean logic consists of three logical operators:
- OR
- AND
- NOT
Each operator can be visually described by using Venn diagrams, as shown below.OR logic
college OR university
Question: I would like information about college.
- In this search, we will retrieve records in which AT LEAST ONE of the search terms is present. We are searching on the terms college and also university since documents containing either of these words might be relevant.
- This is illustrated by:
- the shaded circle with the word college representing all the records that contain the word "college"
- the shaded circle with the word university representing all the records that contain the word "university"
- the shaded overlap area representing all the records that contain both "college" and "university"
OR logic is most commonly used to search for synonymous terms or concepts.Here is an example of how OR logic works:
The more terms or concepts we combine in a search with OR logic, the more results we will retrieve.
college OR university OR campus
For example:
poverty AND crime
Question: I'm interested in the relationship between poverty and crime.
- In this search, we retrieve records in which BOTH of the search terms are present
- This is illustrated by the shaded area overlapping the two circles representing all the records that contain both the word "poverty" and the word "crime"
- Notice how we do not retrieve any records with only "poverty" or only "crime"
Here is an example of how AND logic works:poverty AND crime AND gender
For example:
NOT logic
cats NOT dogs
Question: I want information about cats, but I don't want to see anything about dogs.
- In this search, we retrieve records in which ONLY ONE of the terms is present, the one we have selected by our search
- This is illustrated by the shaded area with the word cats representing all the records containing the word "cats"
- No records are retrieved in the area overlapping the two circles where the word "dogs" appears, even if the word "cats" appears there too
Here is an example of how NOT logic works:Combined AND and OR logic
Question: I want information about the behavior of cats.Search: behavior AND (cats OR felines)
You can combine both AND and OR logic in a single search, as shown above.
The use of parentheses in this search is known as forcing the order of processing. In this case, we surround the OR words with parentheses so that the search engine will process the two related terms as a unit. The search engine will use AND logic to combine this result with the second concept. Using this method, we are assured that the semantically-related OR terms are kept together as a logical unit.
Source: http://www.internettutorials.net/boolean.asp